The present invention relates to the field of tunable optical filters, especially for use in optical communications systems.
High-speed data communications systems need to support the aggregate bandwidth requirements of current and future applications such as supercomputer interconnection, high-quality video conferencing, and multimedia traffic. It has long been clear that these bandwidth requirements can only be met by using optical transmission technologies. Many current approaches favor packet switching and ATM (asynchronous transfer mode) technology, due to their flexibility. The most promising candidate for the future hardware backbone for such networks is dense optical WDM (wavelength division multiplexing), a method of multiplexing a large number of optical data channels on a wavelength basis, i.e. each wavelength is regarded as a different channel, and is routed and manipulated separately from all other wavelengths.
Dense WDM needs advanced optoelectronic components and subsystems, capable of handling the extremely high aggregate bit rates and traffic levels demanded by modern optical data communications systems. One of the most critical components needed for implementation of WDM packet-switched systems is an ultra-fast tunable filterxe2x80x94a wavelength selective element in which the central wavelength of the selected bandpass can be tuned externally and dynamically at a very high rate.
Fast tunable filters are known and available commercially, but the tuning speed of all current known types falls far short of the requirements of future and even of some current optical data transmission systems. The most common optical filters are based on classical interferometers, and include Fabry-Perot and Bragg filters. Such filters are tuned by mechanically moving the resonator structure, and the tuning speed is therefore comparatively slowxe2x80x94typically of the order of milliseconds, or, for the very fastest types, several tons of microseconds.
Another type of tunable filter is based on the Acousto-Optical effect. Such components depend on the interaction between an acoustic wave generated in the device, and the optical signal inputted to the component. Tunability is achieved by altering the frequency of the acoustic wave, which can be simply accomplished by altering the frequency of the electronic signal used to generate the acoustic wave. These filters are, however, polarization dependent, which causes many practical problems. Tuning speeds are reasonable high, of the order of microseconds.
Yet another tunable filter is based on a micromachined semiconductor structure, where the thickness of one of the parts of the structure is altered electrically. Here too, tuning speeds of the order of microseconds can be achieved.
The next generation packet-switched WDM networks are being designed for use with traffic throughputs of the order of Tbits/sec. Such systems therefore require switching and tuning speeds of the order of one nanosecond, and it is evident that even the fastest of the above mentioned filter technologies falls woefully short of these requirements, by about three orders of magnitude.
The present invention seeks to provide an improved high speed tunable optical filter which overcomes disadvantages and drawbacks of existing tunable optical filters, which provides tuning speeds of the order of one nanosecond, and which is capable of implementation is a low cost, high production volume monolithic component.
There is thus provided in accordance with a preferred embodiment of the invention, a tunable optical filter including at least one multiport optical coupler, to one connection port of which is inputted an optical signal, and from another one of which is outputted an optical signal to the end user, and optical transmission line of predetermined length configured as a resonator, with one of its ends connected to yet another one of the connection ports, and its other end connected to still another one of the connection ports, and with a phase modulator inserted in the above mentioned resonator such that the interaction of the phase modulated signal in the resonator with the input signal allows only signals of a preselected wavelength to be transmitted from the output port to the end user.
In accordance with another preferred embodiment of the invention, the output signal can be extracted from the resonator, by means of an additional coupler inserted into the resonator. In this case, the use of three-port couplers is sufficient.
In accordance with a further preferred embodiment of the invention, a filter with variable finesse can be provided by the use of couplers with variable power splitting ratios.
In accordance with yet another preferred embodiment of the invention, there is provided a compound resonator tunable optical filter including at least three multiport optical couplers, to one of the ports of the first coupler is inputted an optical signal, and from one of the ports of the last coupler is outputted a filtered optical signal to the end user, at least two optical resonators of predetermined length interconnecting the free ports of the couplers, and at least one phase modulator inserted in at least one of the above mentioned optical resonators, such that the interaction of the phase modulated signal with the input signal to the first coupler is operative to allow only signals of a preselected wavelength to be transmitted from the output port of the last coupler to the end user.
In applications where an electronic signal is required for the end use, the optical output signal of the filter may be converted into such a signal by means of a fast photodetector mounted on the output port. In applications where an optical signal is required for the end use, such as for optical spectrum analysis, the output optical data signal may be utilized directly.
In accordance with a further preferred embodiment of the invention, there is provided an optical filter including at least one multiport optical coupler, one connection port of the at least one multiport optical coupler receiving an input optical signal, and another connection port of the at least one multiport optical coupler outputting a filtered optical signal, and at least one tunable optical resonator connected to at least one of the at least one multiport optical coupler.
In accordance with yet another preferred embodiment of the invention, there is provided an optical filter as described above and wherein the at least one multiport optical coupler has at least first, second, third and fourth connection ports, the first connection port receiving an optical signal, the second connection port outputting a filtered optical signal and the at least one tunable optical resonator being connected across the third and fourth connection ports.
In accordance with still another preferred embodiment of the invention, there is provided an optical filter as described above and wherein the at least one multiport optical coupler consists of at least first and second optical couplers, each having at least first, second and third connection ports, the first connection port of the first optical coupler receives an optical signal, the second and third connection ports of the first optical coupler are coupled to the at least one tunable optical resonator, the first and second connection ports of the second optical coupler are coupled to the at least one tunable optical resonator, and the third connection port of the second optical coupler outputs a filtered optical signal.
In accordance with another preferred embodiment of the invention, there is provided an optical filter as described above and wherein the at least one tunable optical resonator is operative to select an optical signal with a specific wavelength or to enable the polarization of the filtered optical output signal to be selected.
In accordance with yet a further preferred embodiment of the invention, there is provided an optical filter with variable finesse consisting of an optical element with variable finesse receiving an optical signal and providing a filtered output, and a finesse controller operative to select the finesse of the variable finesse optical element. The variable finesse optical element of this embodiment could consist of an optical coupler with variable power splitting ratio between its connection ports.
In accordance with still another preferred embodiment of the invention, there is provided an optical filter with variable finesse consisting of at least one multiport optical coupler with variable power splitting ratio, one connection port of the at least one multiport optical coupler receiving an input optical signal, and another connection port of the at least one multiport optical coupler outputting a filtered optical signal, and at least one tunable optical resonator connected to at least one of the at least one multiport optical couplers.
There is provided in accordance with a further preferred embodiment of the invention, an optical filter with variable finesse wherein the at least one multiport optical coupler with variable power splitting ratio, has at least first, second, third and fourth connection ports, the first connection port receiving an optical signal, and the second connection port outputting a filtered optical signal, and the at least one tunable optical resonator being connected across the third and fourth connection ports.
In accordance with still another preferred embodiment of the invention, there is provided an optical filter with variable finesse wherein the at least one multiport optical coupler with variable power splitting ratio consists of at least first and second optical couplers, at least one of which has variable power splitting ratio, and each having at least first, second and third connection ports, the first connection port of the first optical coupler receives an optical signal, the second and third connection ports of the first optical coupler are coupled to the at least one tunable optical resonator, the first and second connection ports of the second optical coupler are coupled to the at least one tunable optical resonator, and the third connection port of the second optical coupler outputs a filtered optical signal.
In addition, there is provided in accordance with another preferred embodiment of the invention, an optical filter with variable finesse wherein the at least one tunable optical resonator is operative to select an optical signal with a specific wavelength, thereby providing tunability to both the wavelength and finesse of the optical filter, or enabling the polarization of the filtered optical output signal to be selected.
Additionally, there is provided in accordance with still another preferred embodiment of the invention, an integrated optical filter consisting of at least one multiport optical coupler, one connection port of the at least one multiport optical coupler receiving an input optical signal, and another connection port of the at least one multiport optical coupler outputting a filtered optical signal, and at least one optical resonator connected to at least one of the at least one multiport optical coupler, and wherein at least one of the at least one multiport optical coupler and the at least one optical resonator are formed on an integrated optical substrate.
In accordance with yet another preferred embodiment of the invention, there is provided an integrated optical filter wherein the at least one multiport optical coupler has at least first, second, third and fourth connection ports, the first connection port receiving an optical signal, the second connection port outputting a filtered optical signal, and the at least one optical resonator is connected across the third and fourth connection ports, and wherein at least one of the at least one multiport optical coupler and the at least one optical resonator are formed on an integrated optical substrate.
Additionally, there is provided in accordance with a further preferred embodiment of the invention, an integrated optical filter wherein the at least one multiport optical coupler consists of at least first and second optical couplers, each having at least first, second and third connection ports, the first connection port of the first optical coupler receives an optical signal, the second and third connection ports of the first optical coupler are coupled to the at least one optical resonator, the first and second connection ports of the second optical coupler are coupled to the at least one optical resonator, and the third connection port of the second optical coupler outputs a filtered optical signal, and wherein at least one of the at least one multiport optical coupler and the at least one optical resonator are formed on an integrated optical substrate.
In addition, there is provided in accordance with other preferred embodiments of the invention, an integrated optical filter wherein the at least one optical resonator is a tunable optical resonator, or in which at least one of the at least one optical couplers and the at least one optical resonators includes a discrete non-integrated optical component.
In accordance with still another preferred embodiment of the invention, there is provided an optical filter of the compound resonator type, consisting of at least three optical couplers, each having at least three connection ports, and at least two optical resonators, at least one of which is tunable, each of the at least two optical resonators being connected between two of the at least three optical couplers, and wherein the first connection port of the first optical coupler receives an input optical signal, and the last connection port of the last optical coupler outputs a filtered optical signal.
Additionally, there is provided in accordance with a further preferred embodiment of the invention, an optical filter of the compound resonator type, consisting of at least first, second and third optical couplers each having at least first, second and third connection ports, and at least first and second optical resonators, at least one of which is tunable, each of the at least first and second optical resonators being connected between two of the at least first, second and third optical couplers, and wherein the first connection port of the first optical coupler receives an input optical signal, the second and third connection ports of the first optical coupler are coupled to the first of the at least first and second optical resonators, the first and second connection ports of the second optical coupler are coupled to the first of the at least first and second optical resonators, the third and fourth connection ports of the second optical coupler are coupled to the second of the at least first and second optical resonators, the first and second connection ports of the third optical coupler are coupled to the second of the at least first and second optical resonators, and the third connection port of the third optical coupler outputs a filtered optical signal.
In addition, there is provided in accordance with other preferred embodiments of the invention, an optical filter of the compound resonator type, as described in the previous two paragraphs and wherein the optical resonators consist of loops of optical transmission medium differing in length from each other by predetermined amounts, or wherein this difference in length is controlled by means of a piezoelectric transducer operative to stabilize the length.
In accordance with still another preferred embodiment of the invention, there is provided an optical filter, of the compound resonator type, as described in the previous paragraphs, and wherein at least one of the optical couplers or one of the optical resonators is formed on an integrated optics substrate.
In accordance with yet a further preferred embodiment of the invention, there is provided an integrated optical filter, of the compound resonator type, consisting of at least three optical couplers, at least one of which is formed on an integrated optics substrate, each of the at least three optical couplers having at least three connection ports, and at least two optical resonators, at least one of which is formed on an integrated optics substrate, each of the at least two optical resonators being connected between two of the at least three optical couplers, and wherein the first connection port of the first optical coupler receives an input optical signal, and the last connection port of the last optical coupler outputs a filtered optical signal.
There is further provided in accordance with yet another preferred embodiment of the invention, an integrated optical filter, of the compound resonator type, consisting of at least first, second and third optical couplers, at least one of which is formed on an integrated optics substrate, each of the at least first, second and third optical couplers having at least first, second and third connection ports, and at least first and second optical resonators, at least one of which is formed on an integrated optics substrate, each of the at least first and second optical resonators being connected between two of the at least first, second and third optical couplers, and wherein the first connection port of the first optical coupler receives an input optical signal, the second and third connection ports of the first optical coupler are coupled to the first of the at least first and second optical resonators, the first and second connection ports of the second optical coupler are coupled to the first of the at least first and second optical resonators, the third and fourth connection ports of the second optical coupler are coupled to the second of the at least first and second optical resonators, the first and second connection ports of the third optical coupler are coupled to the second of the at least first and second optical resonators, and the third connection port of the third optical coupler outputs a filtered optical signal.
Additionally, there is provided in accordance with a further preferred embodiment of the invention, an integrated optical filter of the compound resonator type wherein at least one of the optical resonators is tunable.
In accordance with another preferred embodiment of the invention, there is further provided an optical filter wherein the tunable optical resonator is tuned by altering the phase of an optical signal traversing through it by means of a phase modulator.
In addition, there is provided in accordance with yet another preferred embodiment of the invention, an optical filter whose filtered optical output is converted to an electronic signal by means of a photodetector.
There is additionally provided in accordance with yet another preferred embodiment of the invention, an active wavelength division multiplexing system including an optical filter as described in this invention, the filter being operative to select a desired wavelength of an optical signal.
When the tunable optical filter is implemented using bulk fiber optical components, a loop of fiber acts as the tuned resonator and a bulk electro-optical phase modulator is inserted in the loop to provide the variable phase delay which provides the resonator with its tunability. In order to miniaturize the filter to make it compatible with the other integrated opto-electronic components of an optical communications system, and in order to provide the very short loop lengths needed to meet the required specifications of the filter for dense WDM use, and in order to reduce the manufacturing costs of such a filter, the filter can also be implemented on a monolithic integrated optics chip, such as of gallium arsenide, with all of the component parts defined by means of standard semiconductor manufacturing techniques.
The operation of the filters is based on an interferometric interaction between the input optical signal and its delayed produced as a result of the signal traversing the resonator. When the input signal and the delayed signal are in phase at the output port of the last coupler, an output signal is obtained at a specific wavelength, and with the appropriate design, the device operates as an optical narrow bandpass filter. The output spectrum characteristics can be controlled by the phase delay introduced by the phase modulator, so that the device can perform dynamic interferometric processing of the optical signal, creating a dynamic tunable filter. Since optical phase modulation can be performed at exceedingly high rates, the result is an ultra-fast tunable wavelength selective filter. Current technology phase modulators are capable of operation in the 10 GHz range, so that times of the order of one nanosecond are attained. The filter thus offers an attractive solution for the ultra-fast tuning speeds required in Tbit/sec packet-switched WDM networks.